P.A.F. or just PAF is the world's second humbucker guitar pickup, invented by Seth Lover in 1955 as an engineer for Gibson and began use in mass production guitars in 1957. It is commonly mistaken for the first humbucker pickup due to Gibson's popularity and the earlier patent filing date. While the first humbucker pickup in use, and issued a patent, was actually the Gretsch Filter'Tron pickup prototypes, designed by Ray Butts in 1954 at the request of Chet Atkins for his Gretsch 6120.
The strange name of the pickup was not intentional. Gibson and Seth Lover first filed a patent on the design on June 22, 1955. After that, Gibson Les Pauls were equipped with these new pickups, with a sticker on a bottom plate of a pickup that said Patent Applied For. A patent was eventually issued on July 28, 1959. Since it took more than four years to get a patent number, the unnamed pickup had been dubbed "PAF" by many guitarists during the period the application was pending, and this name continued even after the patent had been issued.
PAF pickups can usually be identified by their look: they have two internal coil bobbins under a 1.5" x 2.75" metal cover with one bobbin having a row of six adjustable pole pieces, with the other bobbin having non-adjustable pole pieces. Standard PAF pickups had 5000 or so turns of wire on a bobbin and a DC resistance of 7.5 kΩ.